The concept of folding wing airplanes is commonly associated with fighter aircraft that are stationed onboard aircraft carriers. Because an aircraft carrier has a small flight deck and reduced hanger space compared to ground-based facilities, carrier-based aircraft must have folding wings that reduce their wing span for storage purposes. Providing folding wings on carrier aircraft enables a larger number of planes to be based on a carrier than would otherwise be possible and facilitates rapid movement .of planes between flight deck and hanger bay.
The same kind of space considerations have not previously applied to commercial passenger aircraft or jets. As a result, there has been little or no need to have folding wings on commercial planes. The typical airport served by a passenger jet generally has ample space for aircraft maneuvering on the ground, and for parking and passenger ingress and egress. However, for reasons discussed below, this may now be changing.
Current and projected passenger demographics indicate that the use of larger planes are more economical for commercial airlines to operate. Aircraft size alone, however, is not the only factor controlling how commercial airlines move a given passenger volume from point A to point B in the most efficient manner. The major airlines typically have a fixed number of passenger gates at each major airport. Most major U.S. airports have either been built or renovated during the last twenty or thirty years. During such building process, passenger gates were spaced from each other a distance that was generally considered to be optimal given the size and number of aircraft in use at the time. The present trend toward using larger aircraft is accompanied by problems resulting from significantly larger wing spans than the majority of commercial airplanes in use twenty or thirty years ago. This results in fewer airplanes taking up a fixed amount of parking or berth space around the typical terminal and, consequently, reduces the number of usable passenger gates. The end result is that larger airplanes tend to make it more difficult to accommodate airline scheduling that requires a large volume of flights.
Space restriction is becoming a significant problem at modern airports. The folding wings of carrier-based airplanes, discussed above, suggest one solution to this problem. Applying this military aircraft solution to commercial jets, however, presents several logistical problems in order to achieve the inherently higher desired level of safety.
It is well-known that The Boeing Company, the assignee of the invention disclosed and claimed herein, is presently developing a new model of commercial jet designated as the "777." This jet is larger than Boeing's earlier models and, consequently, has a larger wing span. At least one study done by The Boeing Company estimates that there are approximately one hundred thirty usable gates at six major airports for The Boeing 767 and the DC-10. However, only approximately twenty of these same gates would be suitable for use by the 777, because of its larger wing span.
Providing 777 jets with folding wing tips can enable a larger number of this particular airplane model to be parked or placed around a given terminal at the same time. It has been estimated that nearly as many usable gates would be available for a 777 that is modified in this way as are presently available for the 767 and DC-10.
A 777 with folding wing tips would require a latching hinge mechanism for releasably interlocking the wing tip with the inboard portion of each wing. Safety considerations dictate that such system must have the utmost structural integrity and fail-safe operation. As the skilled person would realize, an aircraft with folding wings presents additional design and safety problems that are not present on aircraft with conventional wings. The wings must be designed so that they do not accidentally fold during flight and that the latching and locking mechanism does not falsely indicate "latched and locked" prior to takeoff when that condition is not true.
In military aircraft, latch pins are used to hold each folding wing in place. These are locked by an independently-operated locking mechanism. The common method used to detect whether or not the latch pins are locked is a wing-mounted red "flag" that is driven above the wing contour by mechanical connections when the latch pins are unlocked. When they are locked, the flag is conversely driven below or flush with the wing contour. The pilot or flight crew determines the proper lock state merely by viewing the position of the flag. For high reliability, the flag is driven directly by the mechanism that locks the latch pins, with a minimum of interconnections. This necessitates physically locating the flag close to the locking mechanism.
Design safety for military aircraft is, by necessity, viewed differently than for civilian aircraft. Military operations typically require optimum airplane performance in order to provide the pilot with the best chance of survival. Military airplane design anticipates that more pilots are lost as a result of combat than equipment failure. Hence, safety considerations are balanced with combat performance considerations. Since combat is not a consideration in the commercial arena, design safety takes on the utmost importance.
The FAA is the government agency that is responsible for closely regulating commercial aircraft design in the United States. Although the FAA is not known to have considered the use of the above-described wing-locking technique used by the military for commercial aircraft, the type of fail-safe operation required by both The Boeing Company and the FAA may not be met. One known drawback to the military design is that the position of the flag does not necessarily provide an indication as to the integrity of the locking mechanism itself. Another drawback to the military design is that it is not particularly well-suited for most modern commercial jets, simply because the wings of such jets are not visible to the flight crew. Therefore, using a similar mechanical flagging technique in connection with the 777 folding wing tip would probably place an unacceptable operating burden on the airline that operates the plane.
The Boeing Company has designed a folding wing tip latching and locking mechanism which meets or exceeds the above-discussed safety considerations in an efficient and cost-effective manner. One aspect of the mechanism is that the hinge of the wing tip is latched into a spread position by a series of latch pins which extend through aligned openings in a clevis and spade connection. Each latch pin is then locked into place by independently-operated primary locks and a mechanically-linked series of secondary locks. The secondary locks are rotated between locked and unlocked positions by an independent hydraulic power drive unit.